This invention relates to the measurement of small magnetic fields produced by the body, and, more particularly, to the measurement of the position of the body as the magnetic field measurements are taken.
The biomagnetometer is a device that measures the very small magnetic fields produced by the human body. The magnetic fields, particularly those produced by the brain and the heart, can be important indicators of the health of the body, because aberrations in the magnetic field can be associated with certain types of disfunctions either for diagnosis or early prediction. Moreover, the magnetic fields produced by the brain are an indicator of thought processes and where such processes occur, and can be used to investigate the mechanisms of thought.
Magnetic fields produced by the body are very small. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over ten million times larger than the magnetic field of the brain.
The biomagnetometer must therefore include a very sensitive detector of magnetic fields. Current biomagnetometers utilize a pickup coil which produces an electrical current when a magnetic field penetrates the pickup coil. The electrical current, which is typically very small in magnitude, is detected by a Superconducting QUantum Interference Device, also known by the acronym SQUID. Spurious effects from the detection of other magnetic fields than those produced by the brain can be removed by appropriate filters. However, the ability of filters to remove all of the extraneous effects is limited. To further improve the signal-to-noise ratio of the system, the subject and pickup coil can be located in a magnetically shielded room. The operation of SQUIDs and their electronics are disclosed in U.S. Pat. Nos. 3,980,076; 4,079,730; 4,386,361; and 4,403,189. A magnetically shielded room is disclosed in U.S. Pat. No. 3,557,777. The disclosures of all of these patents are incorporated herein by reference.
Ultimately, of course, the results of biomagnetic measurements are of most value if they can be associated with their location of origin within the body. To make this association, there are two steps required. The location of origin in space of the magnetic field signal must be determined, and then this location in space must be associated with the position of the body that produces the signal. The location of origin in space can be determined by various modeling, measurement, or analytical techniques. One such approach is the lead field synthesis methodology of U.S. Pat. No. 4,977,896, whose disclosure is incorporated by reference.
The location of the body in space, and thence relative to the location of origin of the magnetic field, was for some period of time accomplished by constraining the subject to a known location. For example, if brain signals were being measured, the head of the subject was held in a known fixed position with head restraints. However, restraints may be uncomfortable and also can result in interfering brain signals as the attention of the subject is focused upon the restraints. An important step forward was made with the discovery of an electromagnetic position sensing device, which is described in U.S. Pat. No. 4,793,355, whose disclosure is incorporated by reference.
The electromagnetic position sensing device utilizes electromagnetic transmitters and receivers to determine the position of the subject's body. For example, a transmitter can be located at a fixed location relative to the sensing coil, and a number of receivers can be fixed to the head of the subject. The signal received by the receivers is analyzed to determine the head position of the subject.
Because the electromagnetic position sensing device itself can generate magnetic fields that are detected by the biomagnetic sensing coil, in some applications the position sensing system is operated intermittently with the taking of biomagnetic data. That is, the electromagnetic position sensing system is operated to determine the head position and turned off. The biomagnetometer is operated to take brain signal data for a period of time and turned off. The two systems continue operating in this intermittent fashion to take data on both head position and magnetic fields produced from the head of the subject. (For more detail, see col. 5, lines 17-30 and col. 13, lines 4-27 of U.S. Pat. No. 4,793,355.)
This intermittent operation approach is completely satisfactory for many situations. However, in others it may be necessary to continue the taking of biomagnetic data for long periods of time, during which the position of the subject may shift. That shift in position may make it impossible to make the association between head position and magnetic field signal. One solution to the problem is to operate the electromagnetic position sensing system continuously at a frequency that is not of interest in the biomagnetic field studies, as described at col. 5, lines 31-38 and col. 13, lines 28-35 of U.S. Pat. No. 4,793,355.
However, there remain some situations where neither of these approaches is fully satisfactory. There therefore exists a need for another approach for determining the position and movement of the body of the subject simultaneously with the taking of biomagnetic field measurements. Such an approach should not interfere with the taking of biomagnetic field data, and should not be obtrusive to the subject so that it would generate spurious brain signals. The present invention fulfills this need, and further provides related advantages.